About reducer's Shuffle JVM Heap Size

[Lijie Xu]

Hi, all.
I'm debugging Hadoop's source code and find an incomprehensible setting.
In hadoop-0.20.2 and hadoop-1.0.3, reducer's shuffle buffer size cannot 
exceed 2048MB (i.e., Integer.MAX_VALUE).  In this way, although*//* 
reducer's JVM size can be set more than 2048MB (e.g., 
mapred.child.java.opts=-Xmx4000m), the heap size used for shuffle buffer 
is at most "2048MB * maxInMemCopyUse (default 0.7)" not "4000MB * 
maxInMemCopyUse".
I think it's not a reasonable setting for large memory machines.
The following code taken from "org.apache.hadoop.mapred.ReduceTask" 
shows the concrete algorithm.
I'm wondering why maxSize is declared as "long" but set as "(int)". I 
point out the corresponding code  with "-->"

Thanks for any suggestion.
---------------------------------------------------------------------------------------------------------------------------
      private final long maxSize;
      private final long maxSingleShuffleLimit;

      private long size = 0;

      private Object dataAvailable = new Object();
      private long fullSize = 0;
      private int numPendingRequests = 0;
      private int numRequiredMapOutputs = 0;
      private int numClosed = 0;
      private boolean closed = false;

      public ShuffleRamManager(Configuration conf) throws IOException {
        final float maxInMemCopyUse =
conf.getFloat("mapred.job.shuffle.input.buffer.percent", 0.70f);
        if (maxInMemCopyUse > 1.0 || maxInMemCopyUse < 0.0) {
          throw new IOException("mapred.job.shuffle.input.buffer.percent" +
                                maxInMemCopyUse);
        }
        // Allow unit tests to fix Runtime memory
-->   maxSize = (int)(conf.getInt("mapred.job.reduce.total.mem.bytes",
-->        (int)Math.min(Runtime.getRuntime().maxMemory(), 
Integer.MAX_VALUE))
-->      * maxInMemCopyUse);
        maxSingleShuffleLimit = (long)(maxSize * 
MAX_SINGLE_SHUFFLE_SEGMENT_FRACTION);
        LOG.info("ShuffleRamManager: MemoryLimit=" + maxSize +
                 ", MaxSingleShuffleLimit=" + maxSingleShuffleLimit);
      }